The use of a portable telephone in a building allows the user a degree of mobility not available from using a telephone which is attached to a telephone jack using a cable. As a result, there is a high demand for portable telephones. Prior art portable telephones have a transceiver and an antenna For transmitting radio signals to and receiving radio signals from a base unit. The base unit is typically coupled to a wireline telephone central office. The base unit also has a transceiver and an antenna for transmitting and receiving radio signals. The radio signal provides the means for communicating between the portable telephone and the base unit, and subsequently to other telephones through the telephone central office.
Even though there is a high demand for portable telephones, there is presently a limit on the number of portable telephones which can be utilized in a building. One of the limitations to the number of portable telephones is that the radio signal frequency spectrum assigned to portable telephones by regulatory agencies is limited. Effort has been made to better utilize this frequency spectrum. As an example, the frequency spectrum is divided into frequency channels having as narrow a band width as practical. Typically, each portable telephone is assigned one of these channels. Thus, the number of portable telephones is limited by the number of assignable channels.
One way to increase the number of portable telephones is to assign more than one portable telephone to the same frequency channel. However, one of the problems of such assignment is that there could be unacceptable interference between the portable telephones using the same channel. Interference occurs when the radio signal transmitted by one portable telephone affects the quality of the signal used by another portable telephone. Interference is especially significant when the intensity of the interfering signal is strong and is of substantially the same frequency as the signal which is interfered with.
One way to reduce interference is by separating the portable telephones using the same frequency channel by a physical structure, such as the flooring separating one floor from another, which attenuates radio signals. If the radio signal passing through the structure is reduced to a low level, it is possible that the interference is reduced to an acceptable level. One way to implement this method is to assign a set of frequency channels having different frequencies to the portable telephones on a floor of a building while the same set of frequency channels is assigned to the portable telephones located on a different floor. The interference between the portable telephones on the same floor is small because the frequencies used by the portable telephones are different. The interference between the portable telephones on different floors would also be small if the physical structure, such as the flooring, attenuates the radio signals passing through the structure to a low level. This method could also be used to allow the portable telephones located in different buildings to use the same set of frequency channels.
The above method has limitations because the physical structure separating the two sets of portable telephones may not be able to sufficiently attenuate the radio signals. Typically, the amount of attenuation to radio signals by a building wall is about 15-25 dB and by a building floor is about 20 dB. In order to avoid interference, the signal intensity of the radio signals on each floor cannot be too strong because of the finite amount of attenuation provided by the walls and the floor. However, a strong signal is required in order to increase the coverage of a portable telephone because there is a direct relationship between the signal intensity and the coverage. The maximum distance between the portable telephone and the base unit is determined by a number of factors. Among them are the separation distance, the physical condition of the building, and the intensity of the signals transmitted by the portable telephones. Typically, the signal intensity required to maintain communication is an increasing function of the separation distance. Thus, in order to cover the distances typically found in a building, the signal intensity may be higher than a level where interference acceptable.
As a result of the above limitations, the number of portable telephones allowed in a building is limited. Consequently, the demand for portable telephones often exceeds their availability. Accordingly, there is a need for an inbuilding communication system, i.e., a communication system which uses a plurality of portable telephones in the building or a portion of the building, which can reduce these limitations thereby allowing more telephones to be employed in the system.